1. Field of the Invention
This invention relates to a colored phosphor, and more particularly to a red colored phosphor employed in a cathode ray tube for color television and to the process for preparing the same.
2. Description of the Prior Art
As is well known in the art, adhesion of blue pigment particles, green pigment particles and red pigment particles to the surfaces of blue emitting phosphor particles, green emitting phosphor particles and red emitting phosphor particles respectively constituting a fluorescent screen of a cathode ray tube for color television markedly enhances the contrast of an image formed on the fluorescent screen. This is because a part of the visible region of the emission spectra inherent to the phosphor is cut out due to the filter effect of pigment particles adhering thereto resulting in a clearer color of emitted light and further because a part of the incident light is absorbed by the pigment particles resulting in reduction of the amount of the light reflected from the fluorescent screen (see Japanese Patent Laid Open No. 56,146/1975). The phosphor with pigment particles adhering to the surface thereof (hereinafter referred to as "pigment coated phosphor") is prepared by mixing a dispersion wherein pigment particles are dispersed in a solution of water soluble resin such as polyvinyl pyrrolidone (PVP) and a dispersion wherein phosphor particles are dispersed in a gelatin solution, stirring the mixed dispersions throughly, and drying the resulting precipitate, as disclosed in, for example, U.S. Pat. No. 3,275,466.
In the pigment coated phosphor, it is important that pigment particles adhere uniformly and firmly to the surface of a phosphor. The pigment coated phosphor prepared by the above-mentioned process is not sufficient in both uniformity in distribution of the pigment particles over the surface of the phosphor and firmness of adhesion of the pigment particles to the surface of the phosphor. When the pigment particles do not adhere uniformly to the surface of the phosphor, but adhere thereto in a state in which they flocculate each other, it is necessary to use a larger amount of pigment particles to obtain a certain relative reflectance (reflectance for the light containing light components of the wavelength ranging from 400 nm to 700 nm in relative to that of MgO plate defined to be 100%) and, as the result, the luminance of the pigment coated phosphor markedly lowers. Further, when the firmness of adhesion of the pigment particles to the surface of the phosphor is not sufficiently high, the pigment particles adhering to the surface of the phosphor are easily separated therefrom even by a small physical friction as in the ball mill treatment. Generally, in the pigment coated phosphor in which the pigment particles do not adhere uniformly to the surface of the phosphor, many of the pigment particles adhere thereto in a state in which they flocculate each other and also, the firmness of adhesion of the pigment particles to the surface of the phosphor is not sufficiently high. Although the uniformity in distribution of the pigment particles over the surface of the phosphor and the firmness of adhesion of the pigment particles to the surface of the phosphor can considerably be enhanced by improving the process for preparing the pigment coated phosphor as disclosed in, for example, Japanese Patent Laid Open No. 5088/1978, the uniformity in distribution of the pigment particles over the surface of the phosphor and the firmness of adhesion of the pigment particles to the surface of the phosphor are always serious problems so far as the pigment coated phosphor is employed. Further, the pigment coated phosphor involves other problems in the production thereof such as requirement for highly sophisticated treatment and long period of operation.
As mentioned above, a pigment coated phosphor in which a phosphor is colored by the pigment particles adhering to the surface thereof has been employed heretofore as a phosphor of a cathode ray tube for color television. Instead of the pigment coated phosphor, a colored phosphor, that is, a phosphor having a body color of its own, can possibly be employed as a phosphor of the cathode ray tube for color television. When employing the colored phosphor, there is no problem of both the uniformity in distribution and the firmness of adhesion of the pigment particles as in the pigment coated phosphor. However, a colored phosphor suitable for a cathode ray tube for color television has not been known heretofore.